Unexplained Friction of 30-Million-Year-Old Sandstone
Sandstone boulders polished so smooth by ancient seas that modern physics can't fully explain their friction properties.
A beach in central France
About 34 million years ago, during the Oligocene epoch, the area now known as Fontainebleau was a shallow, tropical sea. Over millennia, this sea deposited a bed of fine, pure quartz sand nearly 60 meters thick. This deposit, called the Fontainebleau Sands, is remarkably pure, consisting of over 99% silica. Later, as the sea retreated, groundwater saturated with dissolved silica percolated through the upper layers of this sand. This silica-rich water acted as a cement, binding the quartz grains together in a process called silicification.
This process did not happen uniformly. It created vast, flat lenses and tables of incredibly hard sandstone, known as "Grès de Fontainebleau," embedded within the remaining loose sand. Subsequent millennia of erosion, particularly during periglacial periods, wore away the softer, uncemented sand. This action left the harder sandstone concretions exposed, creating the dramatic ridges, plateaus, and scattered boulders seen today. These boulders, some smooth and dome-like, litter the forest floor, resting on the same ancient sand from which they were born.
The paradox of grip
The sandstone of Fontainebleau is a puzzle that challenges the classical understanding of friction. The rock is famously smooth to the touch, polished by ancient water and wind, yet it provides exceptionally high friction for climbers. Standard friction models state that the force of friction is proportional to the roughness between two surfaces. A smoother surface should offer less grip, not more. This paradox is known among climbers and physicists as the "Fontainebleau Problem."
The answer is not simple and remains a subject of scientific inquiry. The unusual friction seems to result from a combination of factors at a microscopic level. The quartz grains are pure but also very fine and well-sorted. The unique silica cementation creates a surface structure that, while feeling smooth, may interact with the soft rubber of climbing shoes in a complex way, maximizing the contact area. This interaction deviates from classical friction approximations, which do not fully account for the deformation of soft materials like rubber against a hard, micro-textured surface. Climbers have long known that conditions like low temperature and wind, which reduce humidity, significantly increase the rock's grip, suggesting that a thin film of moisture is a factor. Despite these observations, a complete physical model that accurately predicts Fontainebleau's friction properties does not yet exist.
💡Fun Facts
The forest of Fontainebleau is widely considered the birthplace of bouldering, a style of rock climbing performed without ropes.
The fine, white sand of the forest was historically mined for manufacturing high-quality glass, including for the famous glassmakers of Murano in Venice.
A bouldering grade system, the "Font scale," originated here and is now used internationally to rate the difficulty of climbs.
Some of the more elaborate, naturally sculpted sandstone concretions in the forest are known locally as "gogottes."
The terrain in the bouldering areas is sandy and uneven with rocks and tree roots, making it generally unsuitable for wheelchairs. Main paths may be more accessible.